570 



CHARLES R. STOCKARD 



Fig. 33 An outline of a short 6 day embrj'o from a strong alcohol solution. 

 The embryo presents a cauda-bifid condition and the normally intra-embryonie 

 blood-forming mass is represented by a densely packed expanse of corpuscles out- 

 side the body of the embryo and spread upon the yolk. 



incorporation of the primitive blood-forming: mesoderm of the 

 yolk-sac into the body of the teleost embryo. 



The situation may be pictured as follows. In the reptiles and 

 birds, for example, the peripheral mesoderm is outspread over 

 the yolk and in it differentiates the blood islands of the area 

 vasculosa. The peripheral mesoderm in Fundulus and teleosts 

 generally does not become outspread over the yolk, but is con- 

 centrated into a median cord or mass within the caudal half of 

 the embryo. Yet even here there is actually a tendency for 

 the cells of this mass to be attracted to the regions of the yolk- 

 sac, and during the early stages of development many cells sepa- 



